Which isn't really good enough. A cryptosystem with a 64-bit key is vulnerable to a brute-force attack (although it isn't trivial). A cryptosystem with a 128-bit key is invulnerable to a brute-force attack, unless the laws of physics change or there's a new model of computation (I've seen claims that quantum computing could reduce a 128-bit key decrypt to two 64-bit key decrypts; I don't know how accurate those are.)
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David ThornleyApr 9 '10 at 14:52

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@David Thornley: 1. I agree. 2. The OP asked if there was a 64 bit type available, so I answered it. I said nothing about how suitable that would be for crypto. 3. My assumption was that it'd be easier to implement 128 bit operations if 64 bit integers are available on your platform.
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Billy ONealApr 9 '10 at 15:21

@samoz: 128-bit arithmetic is far to small for public key crypto. But otherwise I don't know any cryptosystem that uses modular exponentiation. So I'm still wondering what you are trying to implement. I.e., it might well be possible that you could use special purpose modular reduction or some other tricks to improve you implementation.
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abcApr 12 '10 at 11:18